Heater for a diffusion pump



Dec. 10, 1963 D. STEVENSON 3, 7

HEATER FOR A DIFFUSION PUMP Filed June 7, 1961 56 TIMER POWER SOURCE IN VEN TOR DONALD L. STEVENSOIN 4 ATTORNEY United States Patent 3,113,717 WATER FGR A DKFFUSION PUMP Donald L. Stevenson, Albion, N.Y., assignor to Consolidated Vacuum (Iorporation, Monroe Qouuty, N.Y., a corporation of New York Filed June 7, 1961, Ser. No. 115,353 5 Claims. (till. 23lll0l) The present invention relates to difiusion high vacuum pumps and, more particularly, to an improved heater for this type of pump.

Diffusion pumps for producing high vacuum are well known. Such pumps operate on the principle that a liquid having relatively heavy molecules is vaporized in the pump by raising its temperature. The vapor comprising heavy molecules is directed by suitable nozzles in a direction away from the region to be evacuated, towards a mechanical forepurnp. The accelerated molecules of vapor collide against molecules ahead of the nozzle, forcing them toward the mechanical forepump and th reby reducing the pressure within the evacuated region. The vapors are recondensed on a cool wall of the pump where the liquid is permitted to return to the bottom of the pump to be reheated and vaporized.

in diffusion pumps of small and medium size and, in some cases, large size pumps, it has been the general practice to provide a reservoir of oil, for example, at the bottom of the difiusion pump and to apply heat to the bottom of the diffusion pump by means of fiat electric heaters, annular or ring heaters or heaters of the cartridge type which are attached to the bottom of the pump. Thus the oil is caused to boil, producing vapors which rise in the pump wherein they are directed through orifices which produce the proper jets of heavy molecular vapor of the oil. :Such known heating methods, while they may be ellicient tor the types of pumps employed, are inadequate for extra large difiusion pumps which may have pump casing diameter of 30 inches and greater and have heights in excess of 5 feet.

With the enormous sizes of these extra large diliusion pumps, there are relatively great heat losses resulting from the increased area of the nozzle assembly. These heat losses make it ditiicult to supply the nozzle assembly with vapors of suitable quantity and quality with which to provide the desired performance characteristics of a purnp. In order to overcome heat losses from the large surface area of the nozzle assembly, it is necessary to increase the heater input by increasing the number of these units and increase the wattage input for each of them. However, for extra large diffusion pumps, this is not entirely satisfactory because the vapor can still cool slightly due to the lmgeness of the pumps. This cooling allows larger droplets of vapor to form which are in some measure, detrimental to the performance of the pump.

The present invention is particularly adapted for diffusion pumps wherein heat losses are sufficiently high so as to prevent further efficient operation of the pump when the heater elements are at full heating capacity. To this end, there is provided a heater means arranged fully within the hollow nozzle assembly so that heat can be applied to the entire central space of the nozzle assembly. A relatively large hollow heater housing is positioned entrally within the nozzle assembly and rises to a substantial height relative to the total height of the nozzle assembly. Within this housing there is mounted a plu- 3,ll3,7l7 Patented Dec. 10, 1963 ICC rality of heater elements which may be secured to the inner surface of the housing or mounted free-standing therein. These heater elements are adapted to heat the Wall of the heater housing and since this wall is relatively large, the vapor rising from the boiler of the pumps is maintained in a hot or superheated condition so that the nozzle assembly will be supplied with essentially dry hot vapor.

The electrical power supply for the heater elements within the heater housing is provided through a circuit different from that which supplies the boiler heaters for the pump. These heater elements are enengized for operation after vapors have begun to rise from the pump boiler. This will prevent overheating of the heater housing which may result if there is insufficient vapors Within the hollow of the nozzle assembly to dissipate its heat.

A principal object of the present invention is to provide a novel and improved vapor operated diffusion pump.

Another object of the invention is to provide a novel heating device tor a diffusion pump having relatively large size both in diameter and in height.

Another object of the invention is to provide a new vacuum pump operable in the high vacuum range and capable of operating at peak speed and producing greater throughput than is ordinarily possible with a conventional extra large diffusion pump.

These and other aspects of the invention will become apparent from the following specification taken in conjunction with the accompanying drawing.

The single FIGURE is a schematic view of a diffusion pump, embodying the novel heater arrangement of the present invention.

Referring to the drawing, the numeral 10 indicates generally the housing of the diitusion pump which includes a hollow cylindrical casing l2 with an integral bottom wall l4. The bottom wall may be cast, welded or other- Wise secured to the bottom peripheral edge of the hollow cylindrical casing 12 in order to provide a hermetic seal therebetween.

The upper end of the pump is provided with a suitable inlet flange id to which a cooperating flange forming a part of the system being evacuated may be secured. The inlet to the pump is provided with a centrally located opening l8 through the flange l6 and communicates with the interior of the housing ill. The discharge side of the pump is provided with a pipe 2% one end of which opens into the lower portion of the casing 12.

The interior of the pump includes a suitable hollow nozzle assembly indicated generally at 22. The nozzle assembly may be of conventional design comprising a hollow; central sectional nozzle stack having overlapping sections forming annular downwardly directed orifices, such as indicated at 24-, 2e, 23, 3-3 and 32. Vapors rising in the center span of the nozzle assembly 22 are directed out of the respective orifices in downwardly directed nozzles.

As annular spacing 34 between the bottom end 36 of the nozzle assembly and the adjacent portion of the housing 12 serves to permit the flow of condensed oil vapor to return to the bottom of the pump and suitable apertures 38 dormed in the end wall 36 allow this oil to flow into the main pump boiler generally indicated at 4%. The level of the oil in the pump boiler 49* is indicated at ll. It will be apparent that the interior surface of the wall 12 condenses the vapors which are directed downwardly out of the nozzle orifices and the resulting oil droplets run down the wall 12, through the annular spacing 34 and into the main pump boiler 4d.

The boiler 40 may incorporate any suitable type of diffusion pump heater, however, the present invention is particularly adapted for the heaters disclosed in the United States Patents Nos. 2,943,783 and 2,943,784 which are assigned to the same assignee as is the present invention, or for the heater disclosed in the copending application Serial No. 94,727, filed on March 10, 1961, also assigned to the same assignee. For purposes or" illustration, the heater is shown as comprising a plurality of heater cartridges 42 of the type disclosed and claimed in both of the patents referred to above. These cartridge eaters 42 may be positioned in two or more concentric circles about the center axis 44 of the pump housing J. A suitable source of electrical energy do is provided or energizing the cartridges which are connected in parallel thereto. Suitable switches 43, 4 9 are also provided in the circuit to the cartridges for con-trolling energization thereof.

The cartridges 42 for the pump boiler 4% are arranged in concentric circles upon the bottom wall 14. The bottom wall 14 is annular in shape and terminates at an inner circular edge 59 having a radius less than one-half that of the outer periphery of the bottom wall. Hermetically joined to and along this circular edge 59, as by welding, and extending upwardly into the central space of the nozzle assembly is a dome topped, cylindrical, metallic heater casing 52. This casing is preferably thinwalled and extends to a height at least one-half that of the nozzle assembly. Suitable fins 53 are welded or brazed to the casing 52 and extend radially therefrom. Housed within the casing 52 are a plurality of heater elements 54 which may be secured to the inner surface of the casing 52 or mounted in free-standing positions by suitable brackets. As shown in the drawing, the heater elements are preferably mounted upon the surface of the casing.

The heaters 54 are energized by the source of electrical potential 4-6 through conductors 56, and a timer circuit 58 is interposed between the source 46 and the heater elements 54 in order to provide an automatic device for energizing the elements '4 after suflicient time has elapsed for the primary heater elements @3 to have been energized and the generation of vapor within the nozzle assembly has commenced. A switch 60 is connected in this circuit so that the heater 54- may be energized manually if this action is desired. in this event, the timer circuit 58 would be closed for effectively by-passing the same.

In operation, the primary heater elements 42 are energized to bring the pump fluid to a boil and effect vaporization thereof. When vaporization has begun, the heater elements 54 are then energized to bring the pump under full operation. As the vapors rise within the nozzle assembly, those portions which are destined for the upper stages of the pump are heated by the relatively large surface of the heater casing 52 and the fins 53 which in turn are heated by the heater elements 54. In this manner, those portions of the rising vapors are kept hot or even superheated so that the vapors reaching the upper nozzles will be essentially dry and hot for efiicient pumping conditions.

I claim:

,1. A diffusion pump having a pumping area and a boiler area comprising an outer sealed housing having input and output conduit connections thereto and a bottom wall, the bottom wall being common to both the pumping area and the boiler area, a nozzle assembly having a plurality of ascending nozzle stages positioned vertically in the housing and open at the lower end there of, a liquid charge in the housing, heater means arranged on said bottom wall, a first means operatively connected to said heater means for energizing said heater means for boiling the liquid to generate vapor within the nozzle assembly, an auxiliary heater means mounted on said bottom wall and projecting upwardly within said nozzle assembly beyond a plurality of the stages thereof, and a second means c-peratively connected to said auxiliary heater means for enengizing said auxiliary heater means tor heating the vapors within the nozzle assembly.

2. A dilfusion pump having a pumping area and a boiler area comprising an outer sealed housing having input and output conduit connections thereto and a bottom wall, the bottom well being common to both the pumping area and the boiler area, a nozzle assembly positioned vertically in the housing and open at the lower end thereof, a liquid charge in the housing, said open end of said nozzle assembly extending downwardly into said liquid charge, heater means arranged on said bottom wall, a first means operatively connected to said heater means for energizing said heater means for boiling the liquid to generate vapor within the nozzle assembly, an auxiliary heater means mounted on said bottom wall and projecting upwardly within said nozzle assembly for a substantial height therein, said auxiliary heater means including a cylindrical casing and heater elements arranged within said casing for heating the same, and a second means operatively connected to said heater elements for energizinlg said heater elements for heating the vapors rising adjacent said casing.

3. A diliusion pump comprising an outer sealed housing having input and output conduit connections thereto and a bottom wall, a nozzle assembly positioned vertically in the housing and open at the lower end thereof, a liquid charge in the housing, a primary electrical heater means arranged on said bottom wall, circuit means connected to said primary heater means for conducting electrical energy thereto for boiling the liquid to generate vapor within the nozzle assembly, an auxiliary electrical heater means mounted on said bottom wall and projecting upwardly within said nozzle assembly for a distance greater than one-half of the assembly height, a circuit means connected to said auxiliary heater means for conducting electrical energy thereto for heating the vapors within the nozzle assembly, and control means interposed in said last named circuit means for selectively controlling energization of the same after said primary heater means has been energized.

4. A diifusio-n pump having a pumping area and a boiler area comprising an outer sealed housing having input and output conduit connections thereto, an annular bottom wall being formed with an inner circular edge, the bottom wall being common to both the pumping area and the boiler area, a nozzle assembly positioned vertically in the housing and open at the lower end thereof, a liquid charge in the housing, said open end of said nozzle assembly extending downwardly into said liquid charge, a primary heater means arranged along said bottom wall between said inner edge and said housing, a first means operatively connected to said heater means for energizing said heater means for boiling the liquid to generate vapor within the nozzle assembly, an auxiliary heater means mounted on said bottom wall, said auxiliary heater means having a cylindrical casing projecting upwardly within said nozzle assembly for a substantial height therein, said casing having its lower end hermetically sealed relative to said inner edge of said bottom wall, a plurality of heater elements mounted within said casing for heating the same, and a second means operatively connected to said heater elements for energizing said heater elements for heating the vapors within the nozzle assembly.

5. A diffusion pump having a high pressure area and a low pressure area comprising an outer sealed housing having a bottom wall and an input opening in the low pressure area and an output opening in the high pressure area, a nozzle assembly positioned vertically in the housing and open at the lower end thereof, a liquid charge in the housing, a first heater means for boiling the liquid to generate vapor within the nozzle assembly, the first heater means positioned in thermal contact with the liquid change, a first means operatively connected to the first heater means for energizing the first heater means, a secand heater means for heating the vapors within the nozzle assembly, the second heater means projecting upwardly Within the nozzle assembly for a distance greater than one-half of the assembly height, and a second means operatively connected to the second heater means foa' separately energizing the second heater means.

References Cited in the file of this patent UNITED STATES PATENTS Hickman June 26, 1945 Downing et a1 Oct. 9, 1945 Hickman June 24, 1958 FOREIGN PATENTS Great Britain Feb. 10, 196 0 

1. A DIFFUSION PUMP HAVING A PUMPING AREA AND A BOILER AREA COMPRISING AN OUTER SEALED HOUSING HAVING INPUT AND OUTPUT CONDUIT CONNECTIONS THERETO AND A BOTTOM WALL, THE BOTTOM WALL BEING COMMON TO BOTH THE PUMPING AREA AND THE BOILER AREA, A NOZZLE ASSEMBLY HAVING A PLURALITY OF ASCENDING NOZZLE STAGES POSITIONED VERTICALLY IN THE HOUSING AND OPEN AT THE LOWER END THEREOF, A LIQUID CHARGE IN THE HOUSING, HEATER MEANS ARRANGED ON SAID BOTTOM WALL, A FIRST MEANS OPERATIVELY CONNECTED TO SAID HEATER MEANS FOR ENERGIZING SAID HEATER MEANS FOR BOILING THE LIQUID TO GENERATE VAPOR WITHIN THE NOZZLE ASSEMBLY, AN AUXILIARY HEATER MEANS MOUNTED ON SAID BOTTOM WALL AND PROJECTING UPWARDLY WITHIN SAID NOZZLE ASSEMBLY BEYOND A PLURALITY OF THE STAGES THEREOF, AND A SECOND MEANS OPERATIVELY CONNECTED TO SAID AUXILIARY HEATER MEANS FOR ENERGIZING SAID AUXILIARY HEATER MEANS FOR HEATING THE VAPORS WITHIN THE NOZZLE ASSEMBLY. 